1. Field of the Invention
The present invention relates to a vehicle, particularly a lawn mower or a tractor, having a four-wheel steering device.
2. Description of the Related Art
A conventional riding lawn mower having a four-wheel steering device employs a mechanism for steering front and rear wheels in opposite directions at the same time in order to obtain a small turning radius. To realize this mechanism, the riding lawn mower disclosed in U.S. Pat. No. 4,977,733, for example, includes a pitman arm disposed at the left side of a body frame and projecting outwardly from the body frame through an opening formed therein. A front wheel drag link is connected to a front wheel knuckle arm also disposed at the left side of the body frame. A rear wheel drag link is connected to a rear wheel knuckle arm disposed at the opposite side of the pitman arm, i.e. the right side of the body frame. With this construction, the front and rear wheels are steerable in opposite directions.
A region under the body frame is used as a space for accommodating a grass cutting unit. It is therefore difficult to connect the pitman arm and rear wheel drag link in a position below the body frame, with the rear wheel drag link extending transversely under the body frame. Moreover, in order to avoid interference with a rear axle ease, the rear wheel drag link is connected to the knuckle arm which is disposed in a lower position of a rear wheel kingpin assembly. If a large level difference is provided between a point of connection of the rear wheel drag link to the pitman arm and a point of connection thereof to the knuckle arm, the drag link will be subjected to a great bending force due to a reaction from the rear wheels in time of steering, i.e. a reaction from the rear wheels occurring when the pitman arm turns to push the rear wheel knuckle arm through the drag link. In order to minimize the level difference between the point of connection of the rear wheel drag link to the pitman arm and the point of connection thereof to the knuckle arm, the conventional riding lawn mower noted above has the body frame defining the opening for allowing the pitman arm to project therethrough, and the pitman arm and rear wheel drag link are connected to each other adjacent the opening. However, this opening has a drawback of reducing strength of the body frame.
Despite such contrivance, the rear wheel drag link must be bent vertically, and this complicated shape inevitably involves a cost increase. Further, the bending force acting on the drag link as noted above is not very small but requires the drag link to be thick. This results in a drawback of requiting an increased steering effort, for example.